Don't let what I say deter your plans in any way, however take from it what you will.
I too began making plans/ideas on starting up a local fish store, that specialized in Saltwater Fish, Corals, Inverts. I did plan to carry Freshwater fish as well. The building I had found was cheap.. Extremely cheap, at 350 dollars per month for rent. It was spacious over 2000 square foot of store, located in my downtown area. Parking was less then desirable but managable. Projected utility bills were in upwards of 600 a month, Water bill in upwards of 250 a month. And the big hit, was insurance. Insurance was ridiculous, nearly 900 a month. After overcombing the ideas of how much I'd need to maintain the store, then came the initial build fees. To have my local glass company (which i get a 25% discount off their profit margin) to build and setup the 60 30 gallon holding tanks, 4 150 gallon frag tanks, 2 300g display tanks, was going to cost me right around 12,000 dollars. The plumbers estimate to do all the plumbing for the filtration was 3400. The lighting for the store and the tanks was overwhelming. (we never even finished the lighting estimate).
Then there was the task of finding wholesalers willing to work with a new company, and put us on a net 30 business account.
All in all had we continued with the plans the cost of initial setup woulda been in the area of around 35,000 dollars. And thats being quite liberal.
If you do wish to get started, make sure your prepared for alot of heartbreak. The government will not provide you any kind of 'business' grants for this type of business. (as of sept. of last year).
In a city or town where there is already 1 or even 2 or more LFS
's your business is doomed to fail, unless you have the capital to out advertise and out price compete your competitors.
Another thing you have to consider too, during the build phase, is that your tanks must meet certain city codes and pass inspections. Its not like throwing a wooden shelf together and tossing up a bunch of 10g or 20g 'home' aquariums will cut the mustard.
Restrooms have to meet several standards, and be handicap accessible if you provide one for your customers.
Another thing too, is strip malls are usually high in rent, and your limited in many ways to what you can do inside the building.